During the thermal oxidation of a silicon semiconductor wafer using the so-called LOCOS process (LOCal Oxidation of Silicon) first the semiconductor wafer is covered by a barrier layer, such as silicon nitride. The silicon nitride represents a barrier fort he diffusion of oxygen so that the silicon nitride prevents oxidation of the lower lying silicon that may be considered as an example of a semiconductor substrate. At those areas at which an oxidation is to take place the nitride and hence the diffusion barrier is removed prior to the actual oxidation process (for instance, by means of a photolithography step followed by plasma etching). For forming the silicon nitride layer typically deposition processes using a gaseous ambient are used. For this purpose processes exist in which the layer deposition occurs on the front side only, while also processes are known in which the layer deposition is also performed on the rear side. Therefore, the rear side of silicon wafer is either also oxidized (no nitride on the rear side), or is not oxidized at all (rear side is covered by nitride).
Due to the different thermal extension of silicon dioxide relative to silicon the heating of the silicon semiconductor wafer causes mechanical stresses. If the oxide layers are not uniform on the front side and the rear side (for instance, with respect to the thickness or structure), in this case a different mechanical stress at the front side and the rear side is caused, thereby resulting in a disturbing bending of the wafer. Due to these mechanical stresses crystal defects may be generated. A similar situation occurs with respect to silicon nitride and silicon.
In order to avoid a bending of the semiconductor wafer U.S. Pat. No. 5,599,722 discloses to use identical layers on the front side and the rear side (identical thickness, symmetric arrangement). To this end, a LOCOS process is used in which the silicon nitride layer is removed from the rear side prior to the oxidation such that the polysilicon layer located below is also oxidized.
In U.S. Pat. No. 5,849,627 also the same layer thickness is used for the mask oxide, the buried oxide and rear side oxide so as to achieve a balance of the mechanical stresses for bonded wafers during thermal oxidation processes, that is, to reduce the bending of SOI wafers. After the patterning of the front side layer also in this case an imbalance between the front side and rear side is caused.
In U.S. Pat. No. 5,665,631 an SOI manufacturing process is disclosed in which the bending of an SOI wafer is to be corrected by varying the thickness of polysilicon layers (“material layer”) additionally deposited on the rear side. This method, however, requires significant additional effort, since first the additional layer has to be deposited and subsequently this layer has to be removed from the front side.
U.S. Pat. No. 5,837,378 describes a method for avoiding the creation of crystal damage caused by mechanical stresses. For this purpose a LOCOS process is applied to the front side. The nitride layer and the base oxide located below are removed from the rear side. During the subsequent oxidation the front side and the rear side are oxidized. In a variant an additional polysilicon layer is deposited on the nitride layer at the rear side. During the oxidation this polysilicon layer is then oxidized. In both cases it is attempted to create the same thickness on the front side and rear side. The imbalance caused by the patterning of the oxide on the front side is not taken into consideration. When using additionally applied layers further process steps are required, thereby causing additional costs.
A very similar strategy is disclosed in U.S. Pat. No. 5,856,230. On the front side a LOCOS process is performed, on the rear side the nitride and the base oxide are removed prior to the oxidation. The oxidation occurs at the front side and the rear side. Due to the deposition of the rear side layer the mechanical stresses are counter-acted (mainly due to the nitride).
U.S. Pat. No. 5,466,631 discloses a manufacturing technique for SOI substrates, in which the problem of the wafer bending is solved by an appropriate rear side layer. To this end, among others, poly-crystalline silicon may be used that is oxidized or thinned, depending on requirements.
In U.S. Pat. No. 8,801,084 the bending of bonded semiconductor wafers is limited by using compensation, sacrificial and protection layers on the rear side; for example, a sacrificial layer comprised of poly-crystalline silicon is described. During oxidation this layer is “consumed”; the silicon oxide located below and compensating the mechanical stress is not attacked.
In all these publications additional layers are provided on the rear side, requiring additional effort. The patterning of the front side is not addressed. The rear side layer in the form of a non-patterned layer causes significantly higher stresses than the patterned front side layer. Even if the same oxide thickness is created on the front side and the rear side, an imbalance of the mechanical stress and a bending of the semiconductor wafer are caused, at least internal mechanical stresses acting in this way.